For many years, external optical modulators have been made out of electro-optic material, such as lithium niobate. Optical waveguides are formed within the electro-optic material, with metal contact regions disposed on the surface of each waveguide arm. The application of a voltage to a metal contact will modify the refractive index of the waveguide region underneath the contact, thus changing the speed of propagation along the waveguide. By applying the voltage(s) that produce a π phase shift between the two arms, a nonlinear Mach-Zehnder modulator is formed. In particular, the optical signal is launched into the waveguide, and an electrical data signal input is applied to the contacts. The optical signal is phase modulated as it propagates along the arms to generate the output optical signal as a function of the applied electrical data signal input.
Although this type of external modulator has proven extremely useful, there is an increasing desire to form various optical components, subsystems and systems on silicon-based platforms. It is further desirable to integrate the various electronic components associated with such systems (for example, the input electrical data drive circuit for an electro-optic modulator) with the optical components on the same silicon substrate. Clearly, the use of lithium niobate-based optical devices in such a situation is not an option. Various other conventional electro-optic devices are similarly made of materials (such as III-V compounds) that are not directly compatible with a silicon platform.
A significant advance has been made in the ability to provide optical modulation in a silicon-based platform, as disclosed in U.S. Pat. No. 6,845,198 issued to R. K. Montgomery et al. on Jan. 18, 2005, assigned to the assignee of this application and incorporated herein by reference. FIG. 1 illustrates an exemplary prior art silicon-based Mach-Zehnder interferometer (MZI) 1 that is configured to utilize silicon-based modulating devices 2 as described in the above-referenced Montgomery et al. patent. As shown, prior art MZI 1 comprises an input waveguide section 3 and an output waveguide section 4. A pair of waveguiding modulator arms 5 and 6 is shown, where in this example waveguide arm 5 is formed to include a modulating device 2, which may comprise a SISCAP device as disclosed in the Montgomery et al. arrangement, a silicon-based PN junction modulating device, or any other suitable silicon-based modulating arrangement.
In operation, an incoming continuous wave (CW) light signal from a laser source (not shown) is coupled into input waveguide section 3. The CW signal is thereafter split to propagate along waveguide arms 5 and 6. The application of an electrical drive signal to modulator 2 along arm 5 will provide the desired phase shift to modulate the optical signal, forming a modulated optical output signal along output waveguide 4. A pair of electrodes 7, 8 is illustrated in association with modulator 2 and used to provide the electrical drive signals (VREF2, VREF3). A similar modulating device may be disposed along waveguiding arm 6 to likewise introduce a phase delay onto the propagating optical signal. When operating in the digital domain, the electrodes may be turned “on” when desiring to transmit a logical “1” and then turned “off” to transmit a logical “0”.
To the first order, the output power of a conventional MZI as shown above is given by the equation:Pout=Pin/2(1+cos Δφ),where Pout is the output power from the MZI, Pin is the input power, and Δφ is the net optical phase difference between the two arms (e.g., arms 5 and 6 of MZI 1 of FIG. 1). As a result, the optical output power level is controlled by changing the value of the net phase shift φ between the two arms. FIG. 2 is a plot of this relationship, illustrating the output power as a function of phase shift between the two arms (a “1” output associated with maximum output power Pout and a “0” output associated with minimum output power Pout). That is, a differential phase shift between the two arms of the modulator provides either constructive interference (e.g., “1”) or destructive interference (e.g., “0”). Although not shown or described, it is to be understood that in implementation such a modulator may utilize a DC section to optically balance the arms and set the operating point at a desired location along the transfer curve shown in FIG. 2, in this case allowing for a 2-bit data signal to be transmitted during each time period.
There have also been advances in the art of silicon-based optical modulators in terms of utilizing advanced signaling formats. See, for example, U.S. Pat. No. 7,483,597 issued to K. Shastri et al. on Jan. 27, 2009, assigned to the assignee of this application and herein incorporated by reference. As disclosed therein, a multi-bit electrical input data is used and the modulator itself is configured to include at least one modulator arm comprising multiple sections of different lengths, with the total length being equal to a π phase shift. One such exemplary modulator 10 is shown in FIG. 3. Each separate section is driven with a digital logic “1” or a digital logic “0”, that is, digitally driven to either be “on” or “off”, creating the multi-level modulation.
In one embodiment of this arrangement, the modulator sections are optimized in terms of nominal length to provide nearly equal power levels in absolute value, regardless of the position of the section along the modulator arm (i.e., its “position” relative to the cosine-based power curve). Referring again to the transfer function curve of FIG. 2, it is clear that longer length modulation sections can be used to operate at the peak and valley of the cosine curve to provide the same output power change as sections associated with the “steeper”, central area of the transfer curve.
While these arrangements are useful in forming optical modulators that can utilize advanced signaling formats, other less-complicated arrangements may be desirable in certain situations.